A TMR sensor serves as a memory element in Magnetic Random Access Memory (MRAM) devices and in magnetic read heads, and typically includes a stack of layers with a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. In a magnetic read head, the TMR sensor is formed between a bottom shield and a top shield, and the tunnel barrier layer must be extremely uniform in thickness and oxidation state since small thickness variations or slight oxidation differences result in large resistance variations that degrade device performance. In a MRAM device, the TMR sensor is formed between a bottom conductor and a top conductor.
The TMR sensor is also referred to as a magnetic tunnel junction (MTJ) and may have a bottom spin valve configuration wherein a seed layer, anti-ferromagnetic (AFM) layer, pinned layer, tunnel barrier, free layer, and cap layer are sequentially formed on a substrate. The pinned layer has a magnetic moment that is fixed by exchange coupling with the adjacent AFM layer that is magnetized in an in-plane direction. The free layer has an in-plane magnetic moment that is generally perpendicular to that of the pinned layer but is free to rotate 180 degrees under the influence of an external magnetic field generated by passing a current between the bottom shield and top shield in a direction perpendicular to the planes of the MTJ layers. A thin tunnel barrier layer is used so that a current through it can be established by quantum mechanical tunneling of conduction electrons. It is the relative orientation of the magnetic moments between the free and pinned layers that determines the tunneling current and therefore the resistance of the tunneling junction. A sense current detects either a lower resistance (“0” memory state) or a high resistance (“1” memory state) depending on the relative magnetic orientation of the pinned and free layers.
A TMR sensor is currently the most promising candidate for replacing a giant magnetoresistive (GMR) sensor in upcoming generations of magnetic recording heads. An advanced TMR sensor may have a cross-sectional area of about 0.1 microns×0.1 microns or less at the air bearing surface (ABS) plane of the read head. The advantages of a TMR sensor are a higher MR ratio and the preference for current perpendicular to plane (CPP) geometry for high recording density. A high performance TMR sensor requires a low RA (resistance×area) value, high MR ratio, a soft free layer with low magnetostriction (λ), a strong pinned layer, and low interlayer coupling through the barrier layer. The MR ratio is dR/R where R is the minimum resistance of the TMR sensor and dR is the change in resistance observed by changing the magnetic state of the free layer. A higher dR/R improves the readout speed. For very high recording density or high frequency applications, RA must be reduced to about 1 ohm-um2 or less. As a consequence, MR ratio drops significantly. To maintain a reasonable signal-to-noise (SNR) ratio and improved reliability (longer lifetime), a MTJ that provides both of a high MR ratio and ultra low RA value (≦1 ohm-um2) is desirable.
A very high MR ratio has been reported by Yuasa et. al in “Giant room-temperature magnetoresistance in single crystal Fe/MgO/Fe magnetic tunnel junctions”, Nature Materials 3, 868-871 (2004) and is attributed to coherent tunneling. Parkin et al in “Giant tunneling magnetoresistance at room temperature with MgO (100) tunnel barriers”, Nature Materials 3, 862-867 (2004) demonstrated that an MR ratio of about 200% can be achieved with epitaxial Fe(001)/MgO(001)/Fe(001) and polycrystalline FeCo(001)/MgO(001)/(Fe70Co30)80B20 MTJs at room temperature. In addition, Djayaprawira et. al described a high MR ratio of 230% with advantages of better flexibility and uniformity in “230% room temperature magnetoresistance in “CoFeB/MgO/CoFeB magnetic tunnel junctions”, Physics Letters 86, 092502 (2005). However, RA values in the MTJs mentioned above are in the range of 240 to 10000 ohm-um2 which is too high for read head applications. Tsunekawa et. al in “Giant tunneling magnetoresistance effect in low resistance CoFeB/MgO(001)/CoFeB magnetic tunnel junctions for read head applications”, Applied Physics Letters 87, 072503 (2005) found a reduction in RA by inserting a DC-sputtered metallic Mg layer between a bottom CoFeB layer and rf-sputtered MgO. The Mg layer improves the crystal orientation of the MgO(001) layer when the MgO(001) layer is thin. The MR ratio of CoFeB/Mg/MgO/CoFeB MTJs can reach 138% at RA=2.4 ohm-um2. The idea of metallic Mg insertion was initially disclosed by Linn in U.S. Pat. No. 6,841,395 to prevent oxidation of the bottom electrode (CoFe) in a CoFe/MgO(reactive sputtering)/NiFe structure.
Although a high MR ratio and low RA have been demonstrated in MTJs having a MgO barrier layer, there are still many issues to be resolved before such configurations can be implemented in a TMR sensor of a read head. For example, the annealing temperature needs to be lower than 300° C. for read head processing, and rf-sputtered MgO barriers make control of RA mean and uniformity more difficult than with conventional DC-sputtered and subsequently naturally oxidized AlOx barriers. Moreover, a CoFe/NiFe free layer is preferred over CoFeB for low X and soft magnetic properties but when using a CoFe/NiFe free layer in combination with a MgO barrier, the MR ratio will degrade to very near that of a conventional AlOx MTJ. Thus, a TMR sensor is needed that incorporates MgO or other metal oxide barriers without compromising any desirable properties such as high MR ratio, a low RA value, and low magnetostriction.
A barrier layer comprised of TiOXNY and MgO is disclosed in U.S. Pat. No. 6,756,128. In U.S. Pat. No. 6,347,049, MgO/Al2O3/MgO and Al2O3/MgO/Al2O3 are disclosed as tunnel barrier layers having low RA values.
In U.S. Patent Application Publication 2009/0268351, a tunnel barrier is described as having a multilayer structure with individual layers of MgO formed by reactive sputtering of MgO or by natural oxidation of Mg layers.